I updated today (06 Jan 2014), and X11 failed to boot. I tried running startx from a terminal login, and noted that it said that there were no screens found. I trued running nvidia-xconfig, and this failed to resolve the issue. I deleted /etc/X11/xorg.conf, and this got my system up and running on the sucky on-board video, but then trying to re-enable the NVidia card failed. I tried using nvidia-settings, I tried re-installing the driver, I tried rolling it back. None of the above worked.
I am running XUbuntu 13.10-x64 on an Intel Core i7-2600 CPU @ 3.40GHz, 8 GB RAM, most recent updates installed (in fact, that's what broke things).